Фазовая диаграмма системы Ni-Ti

К оглавлению: Другие диаграммы (Others phase diargams)

Ni-Ti (Nickel-Titanium) J.L. Murray The Ti-Ni system is of particular interest because of the shape memory alloys based on TiNi. [55Poo] measured the (bTi) liquidus by a microscopic technique developed for treating reactive, high-melting metals. Microscopic and X-ray methods were used to determine the solidus. The (bTi) solid phase equilibria were examined by [53Mar], [54Mcq], and [55Poo] using metallography, [51Mcq] using the hydrogen pressure technique, and [74Bas] using microprobe analysis of equilibrated two-phase alloys and annealed diffusion couples. The solvus is based primarily on the work of [55Poo]; the (bTi) transus lies above that of [ 51Mcq] and [54Mcq], but slightly below that of [74Bas]. A single equiatomic phase, TiNi, is shown on the assessed diagram. There is no major disagreement concerning the extent of the single-phase region above 900 C. The assessed phase diagram shows a very uncertain eutectoid decomposition of TiNi to Ti2Ni + TiNi3 at 630 с 15 C, although arguments for and against its occurrence have been reported in the literature. [75Per] and [75Was] reviewed the literature on the shape memory effect associated with the martensitic and other metastable transformations of TiNi. The structure changes in TiNi can be classified as (1) the martensite transformation, in which the high-temperature B2 structure is transformed to a monoclinic structure, and (2) "premartensitic instabilities" above the martensite start temperature in the bcc phase. Some concensus has been reached on the structure and stability range of the martensite, but the term " premartensitic instabilities" embraces a variety of imprecisely characterized phenomena. Measurements of start and finish temperatures for the martensitic transformation on cooling and heating have been made. Martensitic start can be influenced by cooling rate and other aspects of sample history. Above the martensitic start temperature (at about 40 to 50 C), a reversible and diffusionless but nonmartensitic transition has been observed. [65Dau] concluded that the transition is a higher order displacive transition based on the continuous distortion of the parent lattice as the temperature was lowered. [67Ber] and [68Ber] found the shape in the heat capacity characteristic of a higher order transition. The structural change has been described in terms of instabilities in lattice displacement waves [82Moi] and also in terms of an additional phase transition to a rhombohedral phase quite independent of the martensitic transition [78Kha]. In terms of lattice displacement waves, [82Mic] showed that most, but not all, of the displacements could be associated with the martensitic transformation. Other waves may lead to the rhombohedral phase. Based on symmetries, neither a tripling of the cubic cell nor a rhombohedral distortion of the cell can be understood as leading to the monoclinic martensite lattice. Therefore, the term "premartensitic instability" should, in this system, be used only to describe softening of phonons associated with the transition to the monoclinic B19-type structure. The precipitation of equilibrium TiNi3 from supersaturated (Ni) at low temperatures is preceded by a coherent metastable phase, g›NiTi3. For an 86 at. % Ni alloy, [74Sin] showed that the precipitation of g›TiNi3 occurs by spinodal decomposition, with simultaneous composition partitioning and ordering. [76Lau] examined early stages of decomposition of a 12 at.% Ti alloy to distinguish between spinodal decomposition that begins as partitioning into solute-rich and solute-poor regions followed by ordering and continuous ordering. They concluded that the former process occurs. This finding does not necessarily contradict [74Sin] because of the composition difference in the alloys used in the two studies. From the Ni-rich side to the Ti-rich side, the sequence of metastable phase boundaries is equilibrium solvus, coherent solvus, spinodal clustering, spinodal ordering. Metastable transition phases leading to the formation of equilibrium TiNi3 are also found on the Ti-rich side of stoichiometry. 51Mcq: A.D. McQuillan, J. Inst. Met., 80, 363-368 (1951). 53Mar: H. Margolin, E. Ence, and J.P. Nielsen, Trans. AIME, 197, 243-247 (1953) . 54Mcq: A.D. McQuillan, J. Inst. Met., 82, 47-48 (1953-1954). 55Poo: D.M. Poole and W. Hume-Rothery, J. Inst. Met., 83, 473-480 (1954-1955). 65Dau: D.P. Dautovich and G.R. Purdy, Can. Metall. Q., 4(2), 129-143 (1965). 67Ber: H.A. Berman and E.D. West, J. Appl. Phys., 38(11), 4473-4476 (1967). 68Ber: H.A. Berman, E.D. West, and A.G. Rozner, NBS Tech. News Bull., 52, 75- 76 (1968). 74Bas: G.F. Bastin and G.D. Rieck, Metall. Trans., 5(8), 1817-1826 (1974). 74Sin: R. Sinclair, J.A. Leake, and B. Ralph, Phys. Status Solidi (a), 26, 285- 298 (1974). 75Per: J. Perkins, in Shape Memory Effects in Alloys, Plenum Press, New York ( 1975). 75Was: R.J. Wasilewski, in Shape Memory Effects in Alloys, Plenum Press, New York, 245-271 (1975). 76Lau: D.E. Laughlin, Acta Metall., 24, 63-68 (1976). 78Kha: V.N. Khachin, Yu.I. Paskal, V.E. Gunter, A.A. Monasevich, and V.P. Sivokha, Fiz. Met. Metalloved., 46(3), 511-520 (1978) in Russian; TR: Phys. Met. Metall., 46(3), 49-57 (1978). 82Mic: G.M. Michal, P. Moine, and R. Sinclair, Acta Metall., 30, 125-138 (1982) . 82Moi: P. Moine, G.M. Michal, and R. Sinclair, Acta Metall., 30, 109-121 (1982) . Published in Phase Diagrams of Binary Nickel Alloys, 1991 and Phase Diagrams of Binary Titanium Alloys, 1987. Complete evaluation contains 11 figures, 7 tables, and 113 references. Special Points of the Ti-Ni System